This patent specification relates to the field of ultrasound information processing systems. In particular, it relates to a method and system for generating extended view ultrasound images.
Ultrasound imaging systems have become increasingly popular for use in medical diagnosis because they are non-invasive, easy to use, and do not subject patients to the dangers of electromagnetic radiation. Instead of electromagnetic radiation, an ultrasound imaging system transmits sound waves of very high frequency (e.g., 2 MHz to 10 MHz) into the patient and processes echoes reflected from structures in the patient""s body to derive and display information relating to these structures.
Conventional ultrasound probes often have a limited field of view compared to the target being imaged, and it is often difficult for a human observer to visualize the whole target from conventional displays of this limited field of view. Responsive to this problem, ultrasound imaging systems have been proposed that create extended view or panoramic images that are more useful in visualizing the target region as a whole. The extended view images are generated by piecing together sequential images taken as the probe is moved in a common plane across the target surface.
One such extended view ultrasound imaging system is discussed in U.S. Pat. No. 5,782,766, which is incorporated by reference herein. According to the ""766 approach, sequential image frames are pieced together by correlating information in the sequential frames to determine a transducer position and orientation associated with each frame, and then using the computed transducer positions and orientations to piece together the sequential frames into an extended view image. The correlation step includes the steps of (i) using a motion detection routine on information in successive images to compute an initial estimation of local motion vectors, (ii) using a fuzzy logic technique to combine the initial estimation with two measurement parameters to derive a final estimation of the local motion vectors, and (iii) applying a least-squares process to the final local motion vectors to estimate global image motion.
The system described in the ""766 patent has several disadvantages due to its use of a correlation algorithm to determine transducer position and orientation. First, the ""766 approach can introduce substantial computational complexity into the imaging process, resulting in reduced system speed, increased system size and complexity, and increased system cost. Second, because of its dependence on inter-frame similarities, the ""766 approach limits the speed of movement of the transducer across the target, and even proposes a xe2x80x9cspeedometerxe2x80x9d for informing the user when the movement is too fast. Third, the ""766 approach is intolerant to tissue motion in the target region, such as the motion of a beating heart, because of its dependence on inter-frame similarities. For similar reasons, the ""766 approach is also intolerant to bumps in the trajectory of the transducer, such as those cause by skin surface irregularities or shakiness in the hand of the user. Moreover, the ""766 approach depends on the user to maintain the transducer""s position and orientation in a common plane as it is moved across the target. If the transducer deviates from that common plane, the correlation algorithm may yield inaccurate results or may completely break down. Even if the correlation algorithm does not break down, the ultimate display may be misleading to the viewer as there may be no way for the ""766 algorithm to detect deviations from the common plane.
Finally, it is believed that the conceptual circularity of the ""766 algorithmxe2x80x94in which relative image content is used to compute the very parameters that are used to xe2x80x9cline upxe2x80x9d that relative image contentxe2x80x94introduces limitations on the output image quality, especially when the user moves the transducer more quickly across the target or guides the transducer away from the common plane. In particular, it is believed that the transducer""s estimated position and orientation has an image-dependent degree of error that can frustrate the increases in output quality that would traditionally be enjoyed by image compounding. Furthermore, this error increases as the user increases the speed of the transducer across the target surface or guides the transducer away from the common plane.
Accordingly, it would be desirable to provide an extended view ultrasound imaging system that has reduced computational requirements, resulting in increased system speed and reduced system size, cost, and complexity.
It would be further desirable to provide an extended view ultrasound imaging system that produces an output image that is robust against increased transducer speed as it is moved across a target surface.
It would be even further desirable to provide an extended view ultrasound imaging system that is tolerant of tissue motion in the target region and tolerant of bumps in the trajectory of the transducer.
It would be still further desirable to provide an extended view ultrasound imaging system that is robust against deviations of the transducer from a common plane as it is moved across a target surface.
It would be even further desirable to provide an extended view ultrasound imaging system that is capable of notifying the user when the transducer has substantially deviated from the common plane, and/or capable of informing the user of the amount of deviation from the common plane.
It would be still further desirable to provide an extended view ultrasound imaging system that constructs the extended view image using a stable algorithm that decreases speckle and increases signal-to-noise ratio.
In accordance with a preferred embodiment, a system for generating extended view ultrasound images is provided in which an ultrasound transducer generates a sequence of scan frames as it is swept across the surface of a target, wherein a position sensor is used to detect the location and orientation of the transducer for each scan frame. The contents of the successive scan frames, together with their location and orientation information, are then processed to generate an extended view ultrasound image of the target region. An output array representing the extended view image is first initialized, and then successively updated as each scan frame is received. In accordance with a preferred embodiment, an alpha-blending algorithm is used to combine the information in the current scan frame with the previous output array values to generate the current output array values.
Advantageously, because the content of the successive image frames is not relied upon to piece them together, the system""s processing requirements are substantially reduced and the output image is robust against increased transducer speed, bumps in the transducer path, and departures of the transducer from a common plane. Moreover, it has been found that the use of an alpha blending algorithm can provide a high quality image with reduced speckle, increased signal to noise ratio, and enhanced contrast resolution even if the spatial precision of the position sensor is less than the spatial resolution of the scan frames themselves.
In accordance with a preferred embodiment, the weighting factor xcex1 is a predetermined value that is used to weight the current scan frame value, while the quantity (1xe2x88x92xcex1) is used to weight the previous output array value, with the current value of the output array being set equal to the sum of the results. Alternatively, the value of xcex1 may be user-adjustable, whereby the user can adjust it closer to 1.0 for increased temporal resolution, e.g., during an extended view scan of a beating heart, or closer to 0.0 for stationary tissue to achieve further decreased speckle and increased signal to noise performance. In alternative preferred embodiments, the weighting factor may be automatically and dynamically adjusted on a per-pixel basis responsive to any of a variety of factors, including location reflectivity, edge motion, or the presence of a systolic cycle versus a diastolic cycle in the patient being imaged.
In accordance with another preferred embodiment, information related to departure of the scan frames from a common plane is computed and displayed to the user. In contrast to prior art systems in which the extended view algorithm breaks down upon substantial departure from a common plane, a system in accordance with the preferred embodiments is not only tolerant to such departure, but can compute the amount of departure and display it to the user for assistance in interpreting the extended view image. Thus, instead of a common plane trajectory, the user may alter the trajectory of the transducer such that the scan planes form a ribbon-like path through the target. The amount of deviation from the common plane may be displayed to the user in a variety of ways, such as by coloring the extended view image on a per location basis to reflect linear departure distances, or by providing a separate graphical display next to the extended view image to reflect the angular departure of the transducer on a per scan line basis.